Refactor the ProcessMonitor API

Refactor the ProcessMonitor API

Refactor the ProcessMonitor API to reduce coupling to
ProcessManager, and stop replicating the whole API and
behavior of ProcessManager.

We introduced an abstract MonitoredProcess class to reduce
coupling, and allow other kinds of external processes
to be monitored too.

Partially Implements: blueprint agent-child-processes-status
Co-Authored-By: Mike Kolesnik <mkolesni@redhat.com>
Change-Id: I0da6071037f9728cc20403324e36c32116bcf00d